1. Field of the Invention
The present invention relates generally to extrusion blow molding machines and, more particularly, to apparatus for variably positioning the die pin with respect to the associated actuator in the accumulator head of an extrusion blow molding machine.
2. Description of the Related Art
Containers and other hollow articles are frequently formed by blow molding. The blow molding process involves providing a tube of heated and softened plastics material (parison) about which a two-piece mold is closed. Air or another gas is introduced under pressure into the parison to expand the parison against the walls of the mold, forming the desired hollow article. Such blow molding machines can be of various types; the most common of which are extrusion blow molding machines and injection-blow molding machines.
In extrusion-blow molding machines, thermoplastic material is heated and softened in an extruder and is conveyed into a die head from which a tubular parison is extruded. The parison can either be continuous, whereby a plurality of molds are sequentially enclosed about successive, axially spaced portions of the parison, or individual parisons can be intermittently extruded and subsequently blown. In the latter instance, the die head is associated with a chamber, usually called an accumulator, in which the extruded material is accumulated to provide a predetermined volume of material. The accumulated volume of material is then ejected through an annular die at the outlet of the accumulator to form a parison having the desired length, diameter, and wall thickness distribution.
Typically, an accumulator has a purging system to accomplish the function of ejecting the accumulated material by forcing it through the die at the outlet. A programming system is provided to control the die opening at the outlet for varying the wall thickness of the extruded parison and to close the die while plastic for the next parison is being fed into the accumulator. It is desirable for the programming system to return rapidly to the close-off position after the parison is formed, so that the flow of melt to the accumulator chamber can be resumed, thereby avoiding a pressure build-up in the extruder. Obviously, movement to die close-off needs to be relatively precise to protect the expensive die tooling; in other words, close the die opening quickly, but in a way that avoids impact between the die pin and outer ring.
Prior art constructions have attempted to provide a rapid return to the die close-off position by various methods. One method involves connecting the piston of the programming cylinder directly to a two-piece rod that attaches to the die tooling, and thereby control the die opening. Spacers are fitted between adjoining ends of the two-piece rod to define a specific rod length that is matched to a particular set of tooling. Since the rod connects directly to the die pin, the length of the rod determines the position at which the piston in the programming cylinder will close off the die gap. The rod is given a length that results in the die gap closing just as the piston reaches the limit of its stroke. Although reasonably effective, this approach requires the time consuming adjustment of spacers in the rod to construct the desired length--which must be performed each time the die tooling is changed.
Another method of the prior art relies on a mechanical stop to enable rapid return to the die close-off position after the parison is formed. This construction also requires that the piston of the programming cylinder connects directly to the die support rod, but further includes a separate element, sometimes referred to as a "stroke limiter." The stroke limiter typically threads onto the upper end of the die rod which allows it to be adjusted to a position where it contacts a stationary (rigid) surface when the die pin is in the close-off position. While this system is also generally effective, the rapid movement of the piston to close off the die gap causes the stroke limiter to "hammer" against the stop surface during every cycle. The continuing impact between these elements results in abnormal wear of the components and necessitates continued adjustment to maintain the desired close-off position.